It's a bit more complicated Chien. One of the interesting things in Sapkowski's stories is that a reader (and Geralt) is often not sure just what (or who) the moster is.
Some "monsters" were made up by people but never existed in reality (nietopyrze). Some were considered "monsters" by people but not by Geralt. And than there were people whom he considered monsters. Last but not least there were "monsters" which Gerald considered as pure legends but which turned out to be real (golden dragon).

In any case, in Sapkowski's books, almost every time Geralt is faced with possible monster killing contract he starts with investigation about what sort of monster (if any) he might be dealing with.